1. Field of the Invention
This invention relates to a method for forming thin films that are used for optical parts, as a reflection preventing film or reflection enhancing film, and especially to a method for forming thin films suitable for use in optical parts applicable in ultraviolet and vacuum ultraviolet regions because they have a good spectroscopic characteristics in the ultraviolet region (wavelength of 230 to 400 nm) and vacuum ultraviolet region (wavelength of 190 to 230 nm).
2. Related Background Art
Methods for forming thin films by sputtering are widely used in the conventional art because they are rather easily used for forming thin film of metals, insulators and various kind of compounds. There are a variety of sputtering methods such as magnetron sputtering and facing sputtering.
The magnetron sputtering, known as a sputtering method capable of a high speed deposition, electrons are confined by magnetic field to increase plasma density.
A method for forming a film of indium-tin mixed oxide (ITO) by sputtering in a gas containing fluorine together with hydrogen and water is disclosed in Japanese Patent Publication No. 6-506266/International Publication WO 92/17620.
In U.S. Pat. No. 4,125,446, a method for forming a metallic film of aluminum by sputtering in a gas containing water and Ar is disclosed.
In a sputtering process for forming oxide films like alumina (Al2O3), alumina or aluminum (Al) is used as a target material and thin films are formed by a sputtering or reactive sputtering in a mixed gas of argon (Ar) and oxygen (O2). In the methods for forming thin films by sputtering or reactive sputtering, a target material is ejected by ions accelerated under an ion-sheath voltage.
When the target material is composed of a compound, for example an alumina (Al2O3) target, the sputtering particles of alumina ejected from the target by an ion impact are decomposed and ejected from the target. The sputtering particles ejected are oxidized by colliding with oxygen or the like, in the plasma or on the surface of substrates.
The method for forming a thin film of aluminum oxide is disclosed in Japanese Patent Application Laid-Open No. 7-70749.
An example of a film-forming apparatus is a sputtering system (an apparatus for forming sputtering thin films) in which an ion source is mounted to enhance reactivity by an ion-assist effect, by irradiating the ions to the substrate.
A sputtering system and method for forming sputtering films are proposed in Japanese Patent Application Laid-Open Nos. 7-258841 and 7-258845, wherein a positive voltage is applied on a target electrode to prevent abnormal electric discharge during sputtering. It was proved that a SiOF film having a lower dielectric constant than that of conventional silicon oxide (SiO2) films is formed when a mixed material of oxides with fluorides is used. For example, it was reported that SiOF films containing F atoms were formed by a plasma chemical vapor deposition (CVD) method by adding a mixed gas containing fluorine.
However, the alumina thin film formed by this method contains unreacted bonds (dangling bonds) that are bond deficiencies, thereby forming a film containing fewer oxygen atoms than required satisfy a stoichiometric oxygen content.
Recently, a so-called eximer stepper, using an eximer laser that emits vacuum ultraviolet light having a short wavelength range (190 to 230 nm), is used for a light source for a projection aligner for producing semiconductor devices to attain a high resolution.
The thin films of alumina (Al2O3) formed by the conventional method for forming thin sputtering films and the system for forming the same described above, has a rather large absorption in the ultraviolet wavelength region and, especially, in the vacuum ultraviolet wavelength region. Therefore, it is difficult to use these thin films as optical thin films for ArF eximer steppers.
Twenty to thirty pieces of lenses are usually combined in the projection optical system for producing semiconductor devices. It is important to form several to dozens of multi-film layers of dielectric materials, as reflection preventing layers, having different refractive indices with each other on the surface of each lens to reduce the reflection index of the lens.
Suppose that 30 pieces of lenses are used in the projection optical system and the absorption of the reflection preventing film on each lens is 1% in the working wavelength region of ultraviolet and vacuum ultraviolet light. Then, the intensity of the transmitted light is reduced to 54.7% (the 60th power of 0.99) of the incident beam since the light transmittance decays in proportion to n-th power (n is the numbers of the lens used) of transmittance. Moreover, when the reflection preventing film has some degree of light absorption, the absorbed light energy is transferred into heat energy, which affects the quality of the printed images due to distortion of the lens. The reflection preventing film at the site where the eximer laser beam focuses may be broken.
An object of the present invention is to provide an optical thin film with little optical absorption and a good spectroscopic characteristics in the ultraviolet region and in the vacuum ultraviolet region.
An another object of the present invention is to provide an optical thin film comprising aluminum oxide containing fluorine.
Still another object of the present invention is to provide an optical film comprising aluminum oxide having the ratio of the number of the atoms other than aluminum to the number of aluminum atoms is larger than 1.55 and smaller than 1.85.
Still further object of the present invention is to provide an optical thin film comprising aluminum oxide prepared by sputtering a target containing aluminum in a gas comprising fluorine atoms in which, if required, oxygen, water and helium are added.